1. Field of the Invention
This invention relates to a stepping motor and, more particularly, relates to an improvement of a stepping motor comprising a stator having magnetic poles, each of said magnetic poles having a winding wound thereon, and a rotor having pole teeth facing said stator magnetic poles through a gap.
2. Description of the Prior Art
FIG. 4 (a) is an expanded view of a conventional stepping motor illustrating the relation positions of the stator and the rotor thereof.
Reference numeral 1 denotes a yoke of the stator. 1-1-1-4 are stator magnetic poles provided on the yoke 1 equidistantly apart from one another. 2-1-2-4 are stator windings, each wound around each of said stator poles. Numeral 3 is the rotor, 3-1-3-5 are pole teeth provided on an outer periphery of the rotor 3, and 4 is a gap between the stator magnetic poles and the rotor pole teeth.
In the conventional stepping motor shown in FIG. 4(a), a tooth width W.sub.s1 of each of the stator magnetic poles 1-1-1-4 is substantially the same as a tooth width W.sub.r1 of each of the rotor pole teeth 3-1-3-5 facing the stator magnetic poles, measuring in a peripheral direction thereof. The number of stator magnetic poles is 4, for example, and they are disposed with an interval x.sub.1 between them. The number of rotor pole teeth is 5, for example, and they are disposed with an interval y.sub.1 between them which is different from said interval x.sub.1. The intervals x, x.sub.1, or x.sub.2 are measured from the center of one stator magnetic pole, around which a coil is wound, to the center of the adjacent stator magnetic pole, around which a coil is wound, and the interval y.sub.2 is measured from the center of one rotor pole to the center of the adjacent rotor pole.
The stator windings 2-1, 2-2, 2-3, and 2-4 are divided into four groups.
In the stepping motor shown in FIG. 4(a), the rotor is rotated stepwise by an angle when an electric current is passed successively through each of the four stator winding groups. The motion thereof will be explained in detail hereunder. FIG. 4(a) shows a state in which the pole tooth 3-1 of the rotor 3 is aligned with said stator magnetic pole 1-1 facing said pole tooth 3-1 through a gap by an electric current flowing through the stator winding 2-1 to generate an electromagnetic force on said stator magnetic pole 1-1. In this state, stator magnetic pole 1-2 and rotor pole tooth 3-2 partially overlap to each other by substantially a half of one tooth width, so that a portion of the pole tooth 3-2 of the rotor 3 projects leftwardly of the stator magnetic pole 1-2 .
Similarly, the stator magnetic pole 1-4 and the rotor pole tooth 3-5 partially overlap each other by substantially a half of one tooth width, so that a portion of the pole tooth 3-5 of the rotor 3 projects rightwardly of the stator magnetic pole 1-4 rightwards. The stator magnetic pole 1-3, however, does not overlap any pole tooth of the rotor 3. In order to move the rotor one step from the state shown in FIG. 4(a), it is sufficient that electric current flow through the stator winding 2-2 and not flow through the stator winding 2-1. By doing so, the stator magnetic pole 1-2 attracts the pole tooth 3-2 of the rotor 3, which opposes and overlaps stator magnetic pole 1-2 by one half of one tooth width, by moving the rotor rightwards by one half of one tooth width until the pole 1-2 and pole tooth 3-2 are fully overlapping.
In order to move the rotor an additional step rightwards, it is sufficient that electric current flow through the stator winding 2-3 and not flow through said stator winding 2-2. By doing so, the stepping motor shown in FIG. 4(a) can be rotated stepwise each time a successive stator winding is energized. Such a driving system is a so called one-phase exciting system, which is the basic driving system.
In such a one-phase exciting system, however, only one set of stator windings is energized at any one time while the remaining sets of stator windings are deenergized. Output, therefore, is relatively low. Accordingly, a two-phase exciting system wherein two adjacent magnetic poles are magnetized at the same time has been used. In the two-phase exciting system, electric currents are sent through the stator windings 2-1 and 2-2 simultaneously as shown in FIG. 4(b). In this state, the pole tooth 3-1 of the rotor 3 is attracted to the stator magnetic pole 1-1 and the pole tooth 3-2 is attracted to the stator magnetic pole 1-2 so that the rotor 3 stops at a position where attracting forces exerted on both of said pole teeth of the rotor 3 are balanced. The relationship between the angular position of the rotor 3 and a torque T of rotation thereof is shown in FIG. 4(c). As shown in FIG. 4(c), the torque of rotation T is generated as a resultant torque of a torque of rotation T.sub.1 generated by the stator magnetic pole 1-1 and a torque of rotation T.sub.2 generated by the stator magnetic pole 1-2, which is larger than that obtained in the case of the one-phase exciting system.
However, employing the conventional stepping motor in the two-phase exciting system as mentioned above, the torques of rotation T.sub.1 and T.sub.2 are in proportion to the electric current value flowing through each winding, and relate to the air gap between the stator magnetic poles and the rotor pole teeth opposing each other so that the rotor 3 stops at a position where the torques of rotation T.sub.1 and T.sub.2 are balanced. Accordingly, the stop position of the two-phase exciting system is generally not as precise as that of the one-phase exciting system. Specifically, if the torques of rotation T.sub.1 and T.sub.2 are different from each other, the resultant torque T can not be passed through the origin of the coordinate in FIG. 4(c), so that positioning errors occur.